Introduction: Sensor Circuits With a MOSFET
You can create a variety of sensing circuits based on voltage divider as the input to the Gate of the MOSFET, this input signal is then amplified by the MOSFET allowing you to interface many devices and even an entirely different circuit.
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
The MOSFET is a voltage operated device this means its mode of operation is dependant of the voltage at the Gate. It has 3 operating regions saturation, linear and cut-off. The MOSFET has a very small linear region and mainly amplifiers operate in this region. Power switching is between the cut-off region and the saturation region.
Gate- The gate for the MOSFET has a very high input impedance in the order of Giga ohms (x10^9) so no need for a protection resistor. the gate has a threshold voltage when it is passed the MOSFET switches and allows current to flow from the positive terminal of the supply into the drain of the MOSFET.
Drain - Essentially the output, on an N-channel MOSFET the drain is at the top, you connect your output here where it may be a motor or an LED for sensitive components don't forget a protection resistor. In a P-Channel MOSFET the drain is at the bottom and connected to the negative rail of the supply. For polarity sensitive components like an LED the cathode is connected to the positive rail.
Source - This pin is connected to 0v.
Double check the datasheet for your MOSFET and make sure that you connect the pins in the correct places otherwise the MOSFET will get really hot and probably break. The MOSFET I will be using is an "STP36NF06 Power switching MOSFET".
The Schematic- Light Sensor
The potentiometer is used to vary the input voltage into the gate, as the light level increases on the LDR the resistance decreases causing the voltage across the potentiometer to increase, this is the voltage that goes into the gate of the MOSFET. The voltage divider formula is V= (Vs*R1) / (R1+R2) where Vs is the voltage of the supply and R1 is the resistor in which you want to find the voltage across.
This Circuit is so that when light level increases the Motor is activated, the LDR and potentiometer can be swapped around so that when light level decrease the motor is activated. (Picture 1 and Picture 2).
* Almost any LDR can be used
* 50k or 47k Potentiometer works best with an LDR in this configuration
* There is a Diode above the Drain to protect the MOSFET against reverse voltage from the Motor, the cathode of the Diode is connected to the positive rail.
* Nearly any power supply can be used, look at a MOSFET datasheet if your are unsure. Make sure the output device you are using can handle the full amount of power from the power supply. (The MOSFET switches all the power from the supply and pumps it through your output device)
Laser Trip circuit
(Picture 3 and Picture 4)
You can make a laser trip circuit by swapping the LDR with the potentiometer so that when light level decrease i.e the laser beam is broken, the motor is activated. Point the Laser beam at the LDR, takes a bit of fiddling to get into position, I used blu-tack.
The protection resistor for the laser can be calculated by R= (Vs-VL) / IL where Vs is the voltage of the supply, VL is the voltage drop across the laser and IL is the amount of current which can safely flow through the laser. Look for the datasheet of your laser, I used a 5mw laser, it draws about 15-20mA and has a voltage drop of about 2.7v.
Adjust the Potentiometer until you find the point where when the laser is broken the motor activates.
You can experiment with the input and output devices, for example a phototransistor which is used to detect infra-red beams, this could be used as the input device and a motor as the output device, point a TV remote at the phototransistor press some buttons and the motor is activated.
You can use these circuits as a foundation and expand on it making it more complex to fulfil a variety of specifications.
Picture 5 shows a range of input and output devices/ components (input components on the top & output components on the bottom) that can be interfaced with a MOSFET. The flexibility of MOSFET and its capabilities make it a popular and useful component in electronics systems.